How did he do it?

When comedian David Walliams, star of Little Britain, made it across the Channel in a pretty spectacular 10 hours and 34 minutes this week, becoming one of the top 50 Channel swimmers of all time, he said: "How did that happen?" Well, partly it happened because of some serendipitous tide action. The shortest distance across the Channel is 21 miles, but most swimmers have to cover around 30 miles to compensate for the ebb and flow of the tides. Not Walliams. For him, the moon was kindly aligned to give him a relatively short journey.

2 Plenty of lubrication

When Captain Matthew Webb became the first person to swim the Channel unaided (in other words without floats, armbands, flippers and so on) in 1875, he did so slathered in porpoise fat. This was to help him keep warm, and to avoid underarm or inner-thigh chafing. Walliams was said to have daubed himself with goose fat (porpoise fat now being out of fashion) before setting off for his charity swim on Tuesday. In fact, the 34-year-old used something called Channel Swimmer's Grease to stop any rubbing or chilling during the estimated 36,000 strokes he took to reach France. A 50/50 mix of lanolin and Vaseline, Channel Swimmer's Grease used to be sold in chemist shops, but is now, by all reports, stocked in just one shop in Britain, in Dover.

3 Blubber

Your average top male swimmer generally has quite a physique: shoulders as wide as the Forth road bridge, a small but perfect bum, and a hairless Action Man-esque chest tapering down to a tight little waist. Walliams has - with the best will in the world - a rather doughy figure. It is more of a rectangle than a triangle, and with no identifiable pectoral muscles whatsoever. Of course, he's not fat. But he is no barrel-chested Adonis, either. So how did he get across so fast, especially given that, by his own acccount, he has "never done anything sporty before"? Well, according to Michael Read, a former Olympic swimmer known as "King of the Channel" since completing a record 33 Channel crossings, extra padding can actually be a bonus on such a marathon swim: "Fat people have a big advantage in that they're better insulated," he says. But then they've also got more weight to drag along, he adds - so it's not all good news for fatties.

4 Snacks

Part of the strategy that allowed Walliams to become the 48th fastest person to cross the Channel (probably - it is slightly too early to be sure as yet, the experts say) was good nutrition before and during his swim. "I have a big bowl of cereal before I set off," says Read. Walliams kept his energy levels topped up throughout his swimming odyssey by having a little snack every half hour. These reportedly included a Milky Way, carbohydrate gel, bananas and some Lucozade. Read himself gets by on water and fructose solutions alone during his crossings. But then he is King of the Channel.

5 The right stroke

There are no rules dictating which stroke must be employed during the crossing, but the fastest time ever, seven hours 17 minutes - managed by one Chad Hundeby, in 1994 - was swum, like Walliams, freestyle (front crawl). That said, a swimmer attempting the challenge just yesterday was planning to do the whole thing in a more sedate breast stroke. And in 2002, Julie Bradshaw bagged a new world record by doing butterfly, by far the hardest stroke, all the way to France in a still-unbeaten 14 hours 18 minutes.

6 Tight trunks

Some cruel souls have laughed at Walliams' Speedos. But he was only following the rules set by the Channel Swimming Association (CSA), the endurance test's governing body. Had he stepped out in more modest Bermudas, or one of those all-in-one leotards favoured by professional indoor swimmers, he would have been disqualified and would now not be able to boast at having been the 666th person to ever complete the challenge. And anyway, says Read, "trunks are far more practical - anything baggier would slow you down and drag in the water". As for the goggles and swimming cap (note the singular - doubling up headgear is strictly forbidden by the CSA), they were nothing fancy. They kept his head warm and the water out of his funny slitty eyes, and that is all that matters.

7 Careful preparation

Walliams was determined not to do a Jade Goody and conk out halfway through, and he knew that some hard graft was in order if he was to make it from Dover to Cap Griz Nez, the bit of France closest to Britain. After all, fewer than 10% of those who attempt the crossing complete the journey, and more people have climbed Everest than beaten the Channel. Walliams took a whole year to prepare himself for the endeavour. He swam in lakes and in the sea whenever possible, and prior to the big day had done a preparatory eight-hour swim in Croatia.

8 Celeb chums

Walliams enlisted the help of former Olympic modern pentathlete Professor Greg Whyte as his trainer. Whyte, a world championship silver medallist who is now director of science and research at the English Institute of Sport, also swam alongside Walliams every two hours during his crossing to help him keep up his pace.

9 Mental strength

There are two kinds of swimmers: those who use their time in the water to forget their earthly troubles and concentrate purely on each stroke; and those who put their brain to work while they are at it. Walliams claimed that he kept his mind occupied on Tuesday by singing Pet Shop Boys, Morrissey and Abba in his head, as well as thinking of how much money he was racking up for Sport Relief. "It's a case of mind over matter," says Read, who before he retired would stave off boredom during long swims by thinking of it as a normal day at the office. "I'd imagine what I'd ordinarily be doing at that time. I'd write to-do lists and things like that."

10 Weather conditions

Walliams was lucky with the tides, and with the weather too. While Tuesday may have been a sticky horrid day for many of us, conditions were just perfect for crossing the channel. There was a light wind - no more than force 3 on the Beaufort scale (you don't start getting white horses and big waves until you get to force 4) - which was blowing helpfully in a north-easterly direction, and though the water was a chilly 15C (59F), at least the air was warm. According to Read, a strong south-easterly wind would have been a disaster, blowing him right back to Dover. Had Walliams had the patience to wait one more day, however, Read says he may have been even quicker. The water yesterday was "still like glass", apparently.